2023ApJ...944...29B

We present the results of a new reverberation mapping campaign for the broad-line active galactic nucleus (AGN) in the edge-on spiral IC 4329A. Monitoring of the optical continuum with V-band photometry and broad emission-line flux variability with moderate-resolution spectroscopy allowed emission-line light curves to be measured for Hβ, Hγ, and He IIλ4686. We find a time delay of ${16.3}_{-2.3}^{+2.6}$ days for Hβ, a similar time delay of ${16.0}_{-2.6}^{+4.8}$ days for Hγ, and an unresolved time delay of $-{0.6}_{-3.9}^{+3.9}$ days for He II. The time delay for Hβ is consistent with the predicted value from the relationship between AGN luminosity and broad-line region radius, after correction for the ∼2.4 mag of intrinsic extinction at 5100 Å. Combining the measured time delay for Hβ with the broad emission-line width and an adopted value of 〈f〉 = 4.8, we find a central supermassive black hole mass of ${M}_{\mathrm{BH}}={6.8}_{-1.1}^{+1.2}\times {10}^{7}$M_⊙. Velocity-resolved time delays were measured across the broad Hβ emission-line profile and may be consistent with an “M”-like shape. Modeling of the full reverberation response of Hβ was able to provide only modest constraints on some parameters, but does exhibit agreement with the black hole mass and average time delay. The models also suggest that the AGN structure is misaligned by a large amount from the edge-on galaxy disk. This is consistent with expectations from the unified model of AGNs, in which broad emission lines are expected to be visible only for AGNs that are viewed at relatively face-on inclinations.